Conventionally, in a vehicle mounted with a turbocharger engine, a study about enhancing the supercharging performance of a turbocharger is carried out. For instance, Japanese Unexamined Patent Publication No. 2014-84757 discloses an engine, in which the flow rate of exhaust gas to be introduced to a turbocharger is controlled by changing the flow area of exhaust gas within an exhaust passage according to a traveling condition of a vehicle to enhance the supercharging performance of a turbocharger.
Specifically, the turbocharger engine described in the Publication is provided with an engine body, an exhaust valve device connected to an exhaust port of the engine body, and a turbocharger connected to the downstream end of the exhaust valve device. The exhaust valve device is provided with a device body including three low-speed exhaust passages arranged in the cylinder array direction and three high-speed exhaust passages arranged in the cylinder array direction; and an exhaust variable valve disposed in each of the high-speed exhaust passages to adjust the flow rate of exhaust gas flowing through each of the high-speed exhaust passages. The device body is constituted by a metal casting body. The low-speed exhaust passages and the high-speed exhaust passages are partitioned into upper and lower two sections by a partition wall extending along the flow direction of exhaust gas.
The exhaust variable valve in the Publication is closed when a vehicle is in a low speed operating range. Closing the exhaust variable valve makes it possible to increase the flow rate of exhaust gas by concentrating exhaust gas of a small amount in the low-speed exhaust passages. This makes it possible to increase a driving force of a turbine in the turbocharger, and to increase an intake pressure. On the other hand, the exhaust variable valve is opened when the vehicle is in a high speed operating range. Opening the exhaust variable valve makes it possible to supply exhaust gas to the turbocharger through both of the low-speed exhaust passages and the high-speed exhaust passages. This makes it possible to drive the turbocharger and to increase an intake pressure while preventing a drawback that the exhaust variable valve becomes an exhaust resistance.
In the turbocharger engine described in the Publication, however, a plate-shaped partition wall located between the low-speed exhaust passages and the high-speed exhaust passages may be distorted due to thermal stress. If the distortion is large, cracks may occur in the partition wall.
More specifically, particularly when a vehicle is in a high speed operating range, exhaust gas passes both through the low-speed exhaust passages and the high-speed exhaust passages, and in addition to the above, the partition wall does not come into contact with external air. Therefore, the partition wall may be heated by exhaust gas both from the upper and lower surfaces thereof, and large thermal expansion may occur in the partition wall. On the other hand, a portion the engine body in the periphery of the partition wall has a larger thickness than the partition wall, has a high rigidity, and the area of the peripheral portion in contact with external air is large. Therefore, the amount of thermal expansion of the peripheral portion is small, as compared with the partition wall. As a result, large distortion may occur between the partition wall and the peripheral portion due to a difference in the amount of thermal expansion, and cracks may occur in the vicinity of a boundary between the partition wall and the peripheral portion.
Further, when the exhaust variable valve is rendered unmovable in a closed state due to thermal distortion of the partition wall and the peripheral portion, or the like, the following drawback may occur. Specifically, when the vehicle is in a high speed operating range, exhaust gas, which should flow through the high-speed exhaust passages, may collide against the exhaust variable valve, and the flow of exhaust gas may stagnate. As a result, exhaust gas may not be smoothly discharged from the exhaust port of the engine body, and a combustion chamber of the engine may cause combustion failure. This may deteriorate the traveling performance in the high speed operating range.